Solid state imagers, including charge coupled devices (CCD) and CMOS sensors, are commonly used in photo-imaging applications. A solid state imager includes a focal plane array of pixels. Each of the pixels includes a photovoltaic device for converting light energy to electrical signals. The photovoltaic device can be a photogate, photoconductor, or a photodiode having a doped region for accumulating photo-generated charge.
Microlenses are commonly placed in a corresponding array over the imager pixel(s). A microlens is used to focus light onto the initial charge accumulation region, for example. Conventional technology forms microlenses from photoresist material which is patterned into squares or circles provided respectively over the pixels. The patterned photoresist material is then heated during manufacturing to shape and cure the microlens.
Use of microlenses significantly improves the photosensitivity and efficiency of the imaging device by collecting light from a large light collecting area and focusing it on a small photosensitive area of the pixel. The ratio of the overall light collecting area to the photosensitive area of the pixel is known as the “fill factor.”
The use of microlens arrays is of increasing importance in imager applications. Imager applications are requiring imager arrays of smaller size and greater resolution. As pixel size decreases and pixel density increases, problems such as crosstalk between pixels become more pronounced. Also, pixels of reduced size have a smaller charge accumulation area. Reduced sizes of pixels result in smaller accumulated charges which are read out and processed by signal processing circuits.
As the size of imager arrays and photosensitive regions of pixels decreases, it becomes increasingly difficult to provide a microlens capable of focusing incident light rays onto the photosensitive regions. This problem is due in part to the increased difficulty in constructing a small enough microlens that has the optimal focal characteristics for the imager device process and that optimally adjusts for optical aberrations introduced as the light passes through the various device layers. Also, it is difficult to correct possible distortions created by multiple regions above the photosensitive area, which results in increased crosstalk between adjacent pixels. Crosstalk can result when off-axis light strikes a microlens at an obtuse angle. The off-axis light passes through planarization regions and a color filter, misses the intended photosensitive region and instead strikes an adjacent photosensitive region.
Microlens shaping and fabrication through heating and melting microlens materials also becomes increasingly difficult as microlens structures decrease in size. Previous approaches to control microlens shaping and fabrication do not provide sufficient control to ensure optical properties such as focal characteristics, radius of the microlens or other parameters needed to provide a desired focal effect for smaller microlens designs. Consequently, imagers with smaller sized microlenses have difficulty in achieving high color fidelity and signal-to-noise ratios.